This invention relates generally to a combination weighing machine and particularly to automatic control of the amount of product fed to each weigher of such a machine.
As well known in the art, a combination weighing machine includes a plurality of weighers for respectively weighing quantities of product and combines the weight values from respective weighers in various fashion to select from the resultant combinations a combination having a total weight satisfying a predetermined condition. It has been known in employing this machine that maximum accuracy of the total weight with respect to the predetermined condition is obtained when the selected combination is composed of the weight values from half of the number of the available weighers. In order to use half of number of weighers, for each weighment, means has been proposed for automatically controlling the amount of product fed to each weigher. For example, U.S. Pat. No. 4,397,364 discloses a device for controlling the amount of feed based upon the detected number of weighers included in each selected combination. U.S. Pat. No. 4,484,645 discloses a device for controlling this amount based upon the detected mean weight from those weighers not included in the selected combination. In these prior art devices, the amounts of feed to all weighers are controlled in a batch based upon a common control data.
However, it has been found that such batch control can not provide intended accuracy due to unavoidable variance in feeding operations of product feeders associated with respective weighers. More particularly, a conventional combination weighing machine includes a product feeding system consisting of a central dispersion table and a plurality of linear feeding troughs radially arranged around the dispersion table. These feeding troughs are subjected to mechanical vibration to convey product to respective weighers at their distal or outer ends. While the amount of feed is controlled by controlling the vibration amplitude, it is difficult to obtain uniform control of amplitude in all feeding troughs. The variance in feeding rate is also caused by difference in surface friction of respective troughs.